Republicans gained control of the US Senate again on Tuesday night by sweeping up seats in West Virginia and Ohio with the help of an unexpected battleground win from Nebraska.

It is worth noting that it is the first time in four years for the Republican party to win a majority in the Senate.

Incumbent GOP Sen Deb Fischer brushed back a surprisingly strong challenge from independent newcomer Dan Osborn.

As tallies rolled in across a map that favoured Republicans, Democrats watched their efforts to salvage their slim majority slip out of reach.

In Ohio, multiple US media outlets projected that Republican Bernie Moreno would defeat incumbent Democrat Sherrod Brown. Those two victories ensured Republicans will hold at least a 51-49 majority in the Senate, with further gains possible as results in other competitive races come in.

Republicans also posted gains as they sought to retain control of the House, which they currently control by a narrow 220-212 majority.

They picked up three seats from Democrats in North Carolina, where they had redrawn district lines to bake in their advantage, while Democrats won control of a Republican-held seat in Alabama that had been redrawn to comply with a US Supreme Court order to create a Black majority district.

Democrats now need to flip at least six seats to take control of the 435-seat chamber.

As in the presidential election, the outcome will likely be determined by a small slice of voters. Fewer than 40 House races are seen as truly competitive.

Republicans stand a chance to widen their Senate majority further if they win in Montana, where Democrat Jon Tester faces a tough reelection battle, and prevail in several competitive Midwestern states. However, they are unlikely to end up with the 60-vote majority needed to advance most legislation in the chamber.